This invention relates to an optical isolator used in an optical system for preventing a light beam emitted from a light source in the optical system from returning to the light source after reflected by an end face of any one of optical elements in the optical system.
Generally, during propagating in an optical system, a light beam emitted from a light source is reflected at an end face of any one of optical elements in the optical system and returns to the light source. For example, in an optical communication system, a laser beam emitted from a laser as a light source is converged by a coupling lens to an end face of an optical fiber. In this event, most of the laser beam enters the optical fiber and propagates therethrough. On the other hand, a part of the laser beam is reflected at the end face of the optical fiber and returns to the laser as a return beam. Generally, the return beam is different in phase and polarizing direction from the laser beam produced in the laser. Therefore, the return beam may often disturb the oscillation of the laser to produce a noise in the laser beam, and, in the worst case, to stop the laser oscillation.
In order to avoid the noise in the laser beam, the return beam must be isolated. To this end, an optical isolator is used. An optical circulator also has a function similar to that of the optical isolator. Basically, the optical isolator is required to be high in isolation characteristic or quenching ratio of the return beam and to be suppressed in transmission loss or insertion loss of an incident light beam.
An existing optical isolator comprises a Faraday rotator of, typically, a thick film of magnetic garnet and a pair of polarizing elements arranged on both sides of the Faraday rotator. Specifically, one of the polarizing elements is arranged at a light-input side of the Faraday rotator and serves as a polarizer while the other polarizing element is arranged at a light-output side and serves as an analyzer. Around the Faraday rotator, a cylindrical permanent magnet such as a rare-earth magnet is disposed and serves as a field-application magnet which generates a magnetic field for magnetizing the magnetic garnet thick film of the Faraday rotator in one direction. An isolator casing made of stainless steel surrounds the permanent magnet. Typically, the above-mentioned optical elements and the permanent magnet are fixed to the isolator casing through a holder by bonding using adhesive, solder, laser welding, or the like.
Most of commercially available magnetic garnet thick films for use as a magnetic material of the Faraday rotator is high in saturation magnetization. Since the field-application magnet is required to have a magnetic field strength sufficient to fully magnetize the magnetic garnet thick film as the Faraday rotator in the one direction, the rare-earth magnet such as a samarium-cobalt (Smxe2x80x94Co) magnet having a high in performance is usually used as the field-application permanent magnet. Since the rare-earth permanent magnet is expensive, the use of the rare-earth magnet for the field-application magnet inevitably increases the cost of the optical isolator as a whole.
As described above, the existing optical isolator inculdes the permanent magnet as the field-application magnet, the holder and the isolator housing which occupy a relatively large space, the optical isolator as a whole is difficult to be reduced in size.
The holder used as the mounting structure of the optical isolator holds the permanent magnet and covers the optical elements for the purpose of reinforcement and protection of the optical elements from external shock. In case where the optical isolator is connected to an optical fiber, another holder having a sleeve is also used for mounting the optical isolator at an end of a ferrule for holding an optical fiber.
In assembling the existing optical isolator which has a plurality of the optical elements, the field-application magnet, the holder and the isolator casing are arranged, much time and labor are required. Therefore, a production cost inevitably becomes high.
Furthermore, in case where the Smxe2x80x94Co magnet, which is a typical rare-earth magnet, is used as the field-application permanent magnet, the Smxe2x80x94Co permanent magnet is required to have a sufficient thickness. This is because the Smxe2x80x94Co magnet is hard and brittle and is therefore difficult in machining. In addition, the holder is required to support the permanent magnet. Therefore, an outer diameter of the optical isolator as a whole becomes relatively large. In this event, the optical isolator can not be assembled in the sleeve holder to be mounted to the ferrule of a small diameter in case where the optical isolator is connected to the optical fiber.
In addition, the optical isolator is often desired to be used in an optical waveguide, an optical device, an optical module, or an optical system. In that case, it is also required for reduction in size and cost. However, the existing optical isolator of the above-mentioned structure is difficult in reduction in size and cost.
It is a primary object of this invention to provide an optical isolator which comprises a high-performance low-cost magnet as a field-application magnet for applying a magnetic field to a Faraday rotator and which can be reduced in size and cost of the optical isolator as a whole.
It is another object of this invention to provide an optical isolator which is easy in assembling and which can readily and effectively be incorporated into an optical component, an optical waveguide, an optical device, an optical module, or an optical system.
According to this invention, there is provided an optical isolator which includes a Faraday rotator of a magnetic garnet thick film and a field-application magnet for generating a magnetic field applied to magnetize the magnetic garnet thick film, wherein the field-application magnet is a selected one of an iron-chromium-cobalt (Fexe2x80x94Crxe2x80x94Co) magnet, a Cunife magnet made of a copper-nickel-iron (Cuxe2x80x94Nixe2x80x94Fe) alloy, a platinum (Pt) alloy magnet made of a Ptxe2x80x94Co alloy or a Ptxe2x80x94Fe alloy, and Cunico magnet made of a copper-nickel-cobalt (Cuxe2x80x94Nixe2x80x94Co) alloy.
In the optical isolator mentioned above, one of the Fexe2x80x94Crxe2x80x94Co magnet, the Cunife magnet, the Pt alloy magnet, and the Cunico magnet is used as an isolator casing.
In the optical isolator mentioned above, the selected one of the Fexe2x80x94Crxe2x80x94Co magnet, the Cunife magnet, the Pt alloy magnet, and the Cunico magnet has a cylindrical shape, a rectangular-frame shape, or a U-frame shape.
In the optical isolator mentioned above, the field application magnet is made of the Fexe2x80x94Crxe2x80x94Co magnet having a dimension such that the relationship txc2x7(S)xe2x88x92xc2xdxe2x89xa70.7 is satisfied where t represents the length of the magnet in the magnetizing direction and S represents a sectional area of the magnet in a plane perpendicular to a magnetizing direction thereof.
In the optical isolator mentioned above, the field-application magnet is made of one of the Cunife magnet, the Pt alloy magnet, and the Cunico magnet which has a dimension such that the relationship txc2x7(S)xe2x88x92xc2xdxe2x89xa70.4 is satisfied.
In the optical isolator mentioned above, the magnetic garnet thick film is a Bi garnet thick film which is formed by at least one of a GdBi garnet film and a TbBi garnet film prepared by liquid-phase epitaxial growth.
In the optical isolator mentioned above, the field-application magnet which is one of the Fexe2x80x94Crxe2x80x94Co magnet, the Cunife magnet, the Pt alloy magnet, and the Cunico magnet satisfies the relationship Hm/4xcfx80Msxe2x89xa70.7 where Hm (Oe) represents the maximum magnetic field generated in a cavity of said magnet and 4xcfx80Ms (G) represents the saturation magnetization for the Bi garnet thick film.
In the optical isolator mentioned above, an isolator casing is made of one of the Fexe2x80x94Crxe2x80x94Co magnet, the Cunife magnet, the Pt alloy magnet, and the Cunico magnet and is used as a holder for holding a plurality of optical elements including the Faraday rotator and a pair of polarizing elements. The optical elements are fixed by an organic adhesive or a solder to the holder in an inclined position such that a light beam is incident to optical element surfaces of the optical elements at an inclination angle not greater than 10xc2x0. Preferably, the holder formed by the isolator casing has a sleeve integrally formed therewith to connect a ferrule for holding an optical fiber.
According to this invention, there is also provided an optical waveguide, an optical device, an optical module, an optical system, or an optical component which has an optical isolator described in any one of the foregoing paragraphs. In a case, the optical isolator preferably has a sleeve integrally formed therewith which is attached to an end of a ferrule for holding an optical fiber.